1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) adaptive channel equalizer, and more particularly to a method of generating address for accessing a coefficient memory and an apparatus employing the same.
2. Description of the Prior Art
In an OFDM method, serially-inputted symbol streams are divided into unit blocks. The symbol streams of each unit block are converted into N number of parallel symbols. After the conversion, these symbols are multiplexed and added by using a plurality of subcarriers having different frequencies, respectively, according to Inverse Fast Fourier Transform (IFFT) algorithm and transmitted via the channel. That is, the N number of parallel symbols are defined as one unit block, and each subcarrier of the unit block has an orthogonal characteristic, which does not have an influence on subchannels. Consequently, in the OFDM method, the Inter-Symbol Interference (ISI) caused by the multi-path fading can be reduced by increasing symbol period in proportion to the number of subchannels (N) while maintaining the same symbol transmission rate as that of a single carrier transmission method. Especially, a guard interval is inserted between the transmitted symbols to enhance the capability of the ISI reduction. As a result, a channel equalizer of simplified structure can be implemented.
The ISI caused by the multi-path fading can be removed with ease by using the guard interval inserted between the transmitted symbols, however, it is not easy to remove the intra-symbol interference, that is, the inter-sample interference, in the OFDM method. Hence, in an OFDM receiver, the channel equalizer for removing the intra-symbol interference is further required. This channel equalizer should detect and remove the distortion generated according to varying channel environment since each sample within a symbol has different subcarriers.
In the OFDM receiver, a Pilot Symbol Insertion (PSI) method can be proposed as an efficient channel equalization method. In the PSI method, a pilot signal is transmitted periodically from an OFDM transmitter and thus time interval for transmitting the pilot signal can be detected in the OFDM receiver. By the way, the transmitted pilot signal is decoded to estimate the distortion caused by the channel environment, in the OFDM receiver. The distortion in symbols belonging to an useful data area can be compensated using the estimated value.
In the PSI method, the pilot signal is periodically inserted in some samples (subchannels) of each symbol. A transfer function of one channel is obtained by using the pilot signal, and transfer functions of another channels are estimated by using an interpolation technique. However, for the OFDM signal, the channel estimation method implemented by the interpolation technique is not capable of adapting to abrupt changes in the channel environment.